Recently, in the fields involving the use of steels, such as construction machinery (e.g., moves and chassis for cranes), tanks, penstocks, and pipelines, the increasing size of structures urges steels to be stronger and also the use environment of such steels has been becoming progressively harsher.
However, strengthening of steels and a harsher use environment are generally known to increase the susceptibility of steels to delayed fractures. For example, in the field of high tensile bolts, JIS (Japanese Industrial Standards) B 1186 stipulates that the use of F11T bolts (tensile strength: 1100 to 1300 N/mm2) should be avoided whenever possible, indicating that the use of high strength steels is limited.
In response to this, methods for manufacturing steels with favorable delayed fracture resistance have been proposed in publications including Japanese Unexamined Patent Application Publication No. H3-243745, Japanese Unexamined Patent Application Publication No. 2003-73737, Japanese Unexamined Patent Application Publication No. 2003-239041, Japanese Unexamined Patent Application Publication No. 2003-253376, and Japanese Unexamined Patent Application Publication No. 2003-321743. These methods are based on various techniques, such as optimization of components, strengthening of grain boundaries, decreasing the size of crystal grains, the use of hydrogen-trapping sites, control of structural morphology, and fine dispersion of carbides.
However, the methods described in the publications listed above, including Japanese Unexamined Patent Application Publication No. H3-243745, Japanese Unexamined Patent Application Publication No. 2003-73737, Japanese Unexamined Patent Application Publication No. 2003-239041, Japanese Unexamined Patent Application Publication No. 2003-253376, and Japanese Unexamined Patent Application Publication No. 2003-321743, do not produce sufficiently strong steels achieving a delayed fracture resistance level that is required in applications where they are exposed to a severely corrosive environment. Thus, steels having both better delayed fracture resistance and a high level of tensile strength, in particular, a tensile strength of 900 MPa or higher, and methods for manufacturing such steels are demanded.
Delayed fractures reportedly occur when hydrogen able to diffuse in steel at room temperature, namely so-called “diffusible hydrogen,” gathers at a stress concentration zone and reaches the threshold limit value of the material. This threshold limit value depends on material strength, its structure, and other parameters.
In general, a delayed fracture of high strength steels starts from non-metallic inclusions, such as MnS, and grows along grain boundaries, such as prior austenite grain boundaries.
Thus, ways of improving delayed fracture resistance include reduction of the amount of non-metallic inclusions, such as MnS, and strengthening of prior austenite grain boundaries.
It could therefore be helpful to provide a high tensile strength steel having delayed fracture resistance better than that of known steels with the tensile strength thereof being 600 MPa or higher, in particular, 900 MPa or higher, as well as a method for manufacturing such a steel.